As semiconductor and MEMS device density increase according commercial demand for more complex, higher performance devices, three dimensional device features often at the base or bottom of a cavity deep within a material object (such as trenches, vias, slits and holes and the like, all referred to herein as “trenches” unless otherwise specified). The base or bottom of a trench can be the material of the body containing the trench or a den object placed in the trench bottom such as an electronic device or substrate or other object to be surface modified.
Increasingly high aspect ratios (depth of the trench to minimum width span diameter therein from a top (proximate) opening to deep into the interior base or bottom (distal) location) have become necessary. The “base or bottom” can be a true base or bottom of the trench or a synthetic bottom such as a shelf or grid within a bench constitutional or supporting a device to be processed but still very deep in the trench in such case the latter is the base or bottom referenced to herein. Aspect ratio is the depth from proximate opening of the trench to its distal base or bottom dividing minimum span of the trench therebetween. Device manufacture involves multiple material deposition cycles combined with lithographic processing of photoresist layers to form patterned masks that protect necessary device layer material while allowing all other material to be selectively removed. Gas phase ion etch reactions, also known as ‘dry etch’ may be used to perform the selective material removal process. Dry etch technology typically involves halogens such as fluorides, chlorides or other reactive species combined with charged particle technology to form reactive radicals. An ion-assisted reaction is a phenomenon in which the incoming ions enhance the surface reactions [1]. The reactive species interact with target material surfaces forming volatile compounds that are released from the surface and pumped away. High aspect ratio dry etch technology is necessitated when physical sputtering of pattern mask material is the limiting factor for high aspect ratio target material removal. In order to effectively process at increasingly high aspect ratios, the following technology performance criteria must be met:
1) Energy Range: Highly controllable flux energies are required so as to minimize sputtering of mask materials while simultaneously enabling delivery of reactive species to high aspect target locations. Typical particle energies for dry etch applications are less than a few hundred eV.2) Process Beam Divergence: As aspect ratios continue to increase either by narrowing of the feature width or increasing the feature depth, the likelihood of beam interaction with the feature sidewall increases dramatically. Bowing is a phenomenon in which the etch profile becomes barrel-shaped because of side etching at the middle of the hole caused by scattered ions hitting the sidewalls [2]. Beam perpendicularity to the feature bottom and low divergence trajectory are necessary to maintain critical feature geometry and feature sidewall shape.3) Surface Charge Accumulation: As layer dimensions continue to be minimized, increasingly stringent voltage budget requirements for device fabrication will be required. Gate oxide breakdown is becoming a serious problem as the gate oxides become thinner [3]. Charge accumulation and capacitive breakdowns through insulating device layers greatly reduce process yields. Additionally, it is extremely difficult to control charge accumulations at the bottoms of high aspect ratio features (holes), causing potential for device damage and repulsion of arriving process species leading to process variability.4) Reactive Species: Reactive gas species must make contact with target material in order to combine and create volatile materials. This is done either of two ways: Either via high concentration background gas incorporation into the process chamber and then irradiation with the energetic beam to form radicals or alternatively, reactive elements are incorporated into the energetic beam for delivery into the high aspect ratio feature.
Traditional charged particle beams and plasmas combined with chemically reactive species have been used to etch material surfaces. However, as feature aspect ratios continue to increase towards 100:1 preferably 200:1 and beyond, traditional charged particle technologies may be unable to meet these stringent process criteria going forward. Beam blowup is the repulsion of particles of like charge from each other, particularly at low energies such as those required for ion beam processing. For this reason, charged particle beams, like plasmas, may have divergent trajectories that can degrade side wall geometries of high aspect ratio features. Surface charging effects on the substrate surface or within the high aspect ratio feature can further exacerbate issues related to charge repulsion effects on sidewall straightness and process repeatability. It is also recognized that the charges associated with ion technologies may ultimately damage device layers due to charge accumulation and capacitive discharging through insulating device layers. Efforts to neutralize the ion beams and substrates by addition of neutralizing electrons add process complexity and are difficult to accurately control in high aspect ratio features (see FIG. 1).